Iraq began a new chapter at midnight on New Year's Day, amid the sporadic flares of small fireworks and the crackle of gunfire. Jan. 1 marks the formal implementation of the Iraqi-U.S. Security Agreement that was labored over for months by officials from the two countries. Rituals for the media and the Iraqi public gave national sovereignty a new face on Thursday morning: a formal celebration marked the handover of Baghdad's Green Zone, the center of the U.S.-aided Iraqi government for the past 5 1/2 years, to Iraqi control. And poles bearing the Iraqi flag were placed prominently at the front gates of shared military bases across the country, symbolizing an Iraqi security force in control. There are new, written ground rules: the U.S. military must now conduct almost all its operations with Iraqi forces, and all arrests must be carried out with warrants issued by Iraqi judges.

But announcing that one is turning over a new leaf is different from actually changing entrenched realities. The question now is whether the new constraints placed on the U.S. military by the security pact will indeed visibly shift leadership to the Iraqis by June  when U.S. combat troops are supposed to be out of Iraq's cities. Another question: What will the consequences of that shift in command responsibility be in terms of security and keeping order in Iraq? (See pictures of a detention center in Baghdad.)

U.S. soldiers say they have already been cooperating with their Iraqi counterparts. "I think there is going to be zero difference between what we do now and Jan. 1 and beyond," said Lieut. Colonel John Vermeesch just before the end of 2008. Vermeesch commands some 880 U.S. combat troops spread across five joint Iraqi-U.S. bases in northwest Baghdad. He is one of the battalion commanders in the 2nd Heavy Brigade Combat Team, 1st Infantry Division, a 4,000-troop force that arrived in Baghdad in mid-October to start a 12-month tour. They say they started complying with the new agreement over a month ahead of schedule, coordinating their movements with Iraqi forces and obtaining arrest warrants from Iraqi judges. Everything else, they say, stays the same.

"Our primary purpose hasn't changed  to secure the population  and will remain that way through the first of January into next year," the brigade's deputy commander, Lieut. Colonel Christopher Beckert, explained a few days before New Year's Eve. "We see the primary threat right now to the Iraqi population as al-Qaeda and also special group criminals. Our plan is to continue to defeat them with our Iraqi counterparts."

That mission may become a greater challenge within the parameters of the pact. Vermeesch and his battalion are based in a former Shi'ite militia hot spot, which promises only more sectarian tensions as Sunni families who fled the town begin to return to their old neighborhoods, which remain dominated by the Shi'a. Attacks are much fewer than last year but still average one a day, Vermeesch says. Now, under the security agreement, U.S. forces can no longer carry out a raid, search, cordon or even a patrol without their Iraqi counterparts. In the past two weeks, four minor operations in Vermeesch's district had to be canceled simply because the Iraqi forces weren't ready for the job.

As he moved from base to base in the final days of December, Vermeesch urged Iraqi commanders to pay more attention to noncombat tasks like equipment maintenance  another fresh responsibility as January gets under way. And even in "combined" patrols, like one Tuesday to search houses for a U.S. soldier's lost weapon, the relationship remains unbalanced at best: a handful of Iraqi police acted as little more than chaperones for a U.S.-led search.

In making arrests, there are other challenges. Previously, U.S. troops could detain Iraqi nationals at will, and for unspecified lengths of time without due process if they were suspected to have participated in insurgent activity. Now the troops must first obtain an Iraqi warrant  something that often requires the presentation of a witness to a judge. Even once a suspect is detained, U.S. forces are legally obligated to hand him to Iraqi forces within 24 hours.

Brigadier General Kadre Abdel Latif, the director of the Interior Ministry's criminal investigations branch in northwest Baghdad, views this as a frustrating new burden  and one the Iraqis aren't ready for. "The army and police  especially the army  impedes our efforts in arresting these [wanted] people," he told Vermeesch during a visit this week. Once information about a target is distributed to Iraqi forces, "the information gets to the wanted people, and they take off." Latif doesn't like the idea of detainees being handed over to Iraqi forces either: they are often released before they can be prosecuted.

And then there is the trouble of investigations. Some U.S. commanders expressed irritation with a continuing tendency of Iraqi forces to "contaminate" the scene of a bombing by removing valuable evidence and witnesses before U.S. troops get there. But with the combined-operation requirement, there is little the Americans can do about it.

Some Iraqi government officials tie the behavior of Iraqi troops to corruption, incompetence and party loyalties that continue to permeate the ranks. In some areas, tensions and mistrust, though once far stronger, still linger between the Iraqi army and the police. One of the Iraqi army commanders Vermeesch visited recently had been involved in a shoot-out with police forces just 10 months before. Some army commanders are not shy about complaining about the police. "I am very sure that the Iraqi Army is equipped and ready to handle the situation after Jan. 1," said another commander, Col. Abbas al-Moussawi. "We're not sure MOI [Ministry of the Interior, a reference to Iraqi police] forces are ready."

The U.S. military will face real hurdles as it pressures the Iraqis to carry out their end of the now formalized bargain. But not all the constraints laid out in the security pact are binding. A commanding coalition general still wields the power to authorize any operation unilaterally, and U.S. troops don't need to consult the Iraqis if responding to an imminent threat or in self-defense  a provision so broad that most of the limits on U.S. troop operations could effectively be bypassed if commanders deem it necessary to do so. "I would not view them as loopholes," Vermeesch says. "I would view them as provisions to allow U.S. forces in everyday situations to protect themselves."